1. Field of the Invention
The invention relates to a pressure relief valve for pressure equalization between a closed space and the surrounding atmosphere, and particularly to such pressure relief valves used in tankers for oil products and chemicals. In and in the following explanation and description this field of use will be taken as point of departure, but it will be understood that the same principles will be equally applicable to other fields of use, e.g., for stationary storage tanks or for rolling transportion tanks for liquid petroleum products or chemicals.
2. Prior Art
When a certain volumetric quantity of product per time unit, e.g., measured in m.sup.3 /h, is loaded into a tank, the associated pressure relief valve is to be opened when the pressure in the tank has risen above a pre-set closing pressure, and thereafter a quantity of gas is to be blown off per time unit corresponding to the quantity of product loaded per time unit with an addition, where applicable, for evaporation from the liquid surface in the tank. The blow-off quantity per time unit depends on the flow resistance of the valve--and thereby on the lifting height of the valve--and on the pressure in the tank. The maximum blow-off quantity per time unit--with deduction of any addition for evaporation--which is obtainable without the pressure in the tank exceeding a certain safety limit is referred to as the capacity of the pressure relief valve, which is thus a measure of the maximum permissible loading quantity per time unit (e.g. m.sup.3 /h).
The volumetric quantity of product loaded per time unit is referred to as the loading rate (e.g. m.sup.3 /h), and the volumetric quantity of gas blown off per time unit as the blow-off rate (likewise e.g. m.sup.3 /h). The latter is not to be confused with the blow-off velocity, which means the linear velocity at which the gas is blown off through the valve opening, e.g., measured in m/sec..
More particularly, the invention relates to a pressure relief valve which includes a valve housing provided at its top with a blow-off opening formed with an upwardly-facing, substantially conical valve seat, a drop-shaped body suitable for concentrating a flow of gas impinging on its underside to an upwardly-directed jet, a substantially conical valve face for closing against the valve seat being formed on the downwardly-facing portion of the rounded surface of the drop-shaped body, which thereby constitutes a valve body, and a lifting disc connected with the drop-shaped body and located at a lower level within the housing, the lifting disc having an outer diameter greater than the inner diameter of the valve seat, stop means for defining a fully open position of the valve, the lifting disc being surrounded by a free passage slot in the closed position of the valve.
Such a pressure relief valve is disclosed and claimed in international patent application PCT/DK90/00050, published on 07.09.90 under No. WO 90/10168.
Due to the arrangement described, the lifting pressure is shifted at the initiation of the opening from the smaller area of the underside of the drop-shaped member within the valve seat to the larger area of the lifting disc, whereby a higher lifting speed is obtained.
In the embodiment disclosed in WO 90/10168, the valve housing has an inner configuration such that the free passage area around the lifting disc is increased when the drop-shaped body and the lifting disc are lifted, whereby also at increasing blow-off quantity, a lower pressure is obtained than would otherwise be possible, until the valve approaches its fully open condition.
It has been found, however, that in some circumstances, depending, i.e., on the type of product to be loaded and the facilities and practices of the loading station, situations may occur, particularly at relatively low rates of loading and correspondingly low flow-off rates from the tank being loaded, where the flow-control element consisting of the drop-shaped valve body and the lifting disc will be subjected to fluctuations of the rate of gas flow caused, e.g., by turbulence or non-uniform distribution of the flow. The resulting vibratory movement of the flow-control element will act back on the rate of flow, and thereby a reciprocal-amplifying effect may be initiated which may cause the valve to vacillate forth and back between the fully open and the closed position. Thereby the valve may be subjected to a rapid succession of heavy clashes of metal against metal, which is in itself undesirable for mechanical reasons, and besides may produce a loud noise, which may even be amplified by the tank wall as a reverberator.